Spectroscopy, such as rotational spectroscopy, is a powerful structural tool in physical chemistry. For example, the relationship between the molecular structure and the rotational transition frequencies can be used for structure determination of gas phase samples. Other effects in the rotational motion of molecules, such as centrifugal distortion, hyperfine spectral structure from quadrupolar nuclei, or frequency shifts caused by tunneling motion, can be used to provide further characterization of the molecular structure and low frequency vibrational motions.
Fourier transform spectroscopy in the millimeter (mm) wave or terahertz (THz) frequency region is particularly useful for chemical detection and characterization. Typical approaches to generate useful frequencies from common sources (e.g., Digital-to-Analog converters) involves the use of frequency mixers, where phase coherence between the excitation source and a frequency reference source of the frequency mixer can be challenging to attain. There is hence an unmet need for approaches that permit for phase reproducibility during acquisition in Fourier transform millimeter wave spectroscopy.
Additionally, signal generation in Fourier transform spectroscopy typically employs the use of a fixed frequency source, such as a clock, to trigger signal generation. This is needed to maintain phase reproducibility from one acquisition to the next, but limits and/or otherwise impedes the ability of the signal generator to receive trigger signals and/or events originating from events that do not necessarily occur in a fixed-frequency manner or setting. Accordingly, there is an unmet need for approaches that permit synchronization of signal generation and/or acquisition to external, sometimes asynchronous, events.